What are complementary therapies? There is a long list - the Institute for Complementary Medicine (ICM), which provides register of practitioners, cites more than 60 disciplines.

They range from the fairly traditional, such as sports massage and osteopathy to the more esoteric, such as Pranic healing, in which the practitioner "interacts with the patient's aura or energy field".

Among the more widely used complementary therapies are homeopathy, in which minute traces of a remedy are massively diluted; the foot massage-based reflexology; hypnotherapy; and treatments based on traditional Chinese medicine such as acupuncture.

Do they work? This is the key question and the battle lines were drawn a long time ago. Sceptics insist there is no scientifically rigorous evidence that they are effective, and that complementary therapies are based on a series of diverging and seemingly contradictory pseudo-scientific models.

Proponents argue that there is compelling anecdotal evidence from patients that many treatments do work, notably in palliative care where the symptoms of a disease are treated rather than a cure sought. They also stress that it is not meant to replace traditional medicine.

"It is important to call them complementary therapies, not alternative therapies, as they are not meant to replace conventional medicine," says ICM president Frances Fewell.

Are they available on the NHS? Yes. Homeopathy has been available on the NHS since its inception in 1948, but in practice it varies. GPs can refer patients for complementary therapies, which the NHS funds, but it is up to them to decide what is appropriate.

There is no "absolute right" to complementary treatment on the NHS, the Department of Health website notes, with decisions being based on medical and funding criteria.

A recent survey of English GP partnerships found that nearly 40% now provide some complementary treatments. However, much of this figure is taken up with more mainstream therapies such as osteopathy, chiropractic and massage. Hospital referrals depend on individual NHS trusts.

Are they widely used by the NHS? Increasingly so, especially for pain relief and other palliative care, with cancer treatment often at the forefront.

Mrs Fewell of the ICM says the divisions between conventional and complementary medicines are becoming increasingly blurred.

"Increasing numbers of nurses and midwives are regularly using what I would term 'hands-on' care like massage and reflexology as part of their work," she says.

"With GPs, a lot of the change is being led by patients asking for different types of treatments, often for conditions that perhaps would not require traditional medical intervention."

While many patients are referred to outside practitioners, the therapies are sometimes integrated into the mainstream NHS.

For example, Professor Michael Baum, who organised the doctors' letter, criticised the £20m of public money being spent on the Royal London Homeopathic hospital, which was integrated into University College hospital in 2002.

Glasgow's Gartnavel hospital, meanwhile, has a homeopathic unit containing a 15-bed ward, the only such facility in Britain.

Do doctors accept complementary medicines? Again, it depends, although attitudes are changing fast. Mrs Fewell says: "I can remember GPs saying that if a patient went to see an osteopath they would not be welcome back. For the most part it's very different these days."

Complementary therapies are used increasingly every year, and in ever-closer liaison with the NHS. But as the doctors' letter proves, many sceptics remain.